Previously, comment migrations relied on the d6_node_type and d7_node_type source plugins to determine which comments and comment fields to migrate.
Unfortunately, this did not take into account that the comment module may be enabled on the D8|9 destination site but was disabled on the source site. Consequently, this would generate migrations for comments and comment fields that simply did not exist in Drupal 6|7.
To solve this, a new migration source plugin was added: comment_type. Any place where you used to use d6_node_type or d7_node_type in a comment-related context will now benefit from using comment_type instead.
It is recommended but not required to use comment_type instead of d6_node_type or d7_node_type: his will ensure only actual D6|7 comment types will be migrated. Existing migrations will continue to work as-is without making any changes.
